The present invention relates to papermaking, and more particularly to an apparatus and process for removing water from a cellulosic web.
Cellulosic fibrous structures, such as paper towels, facial tissues, napkins and toilet tissues, are a staple of every day life. The large demand for and constant usage of such consumer products has created a demand for improved versions of these products and, likewise, improvement in the methods of their manufacture. Such cellulosic fibrous structures are manufactured by depositing an aqueous slurry from a headbox onto a Fourdrinier wire or a twin wire paper machine. Either such forming wire is an endless belt through which initial dewatering occurs and fiber rearrangement takes place.
After the initial formation of the web, which later becomes the cellulosic fibrous structure, the papermaking machine transports the web to the dry end of the machine. In the dry end of a conventional machine, a press felt compacts the web into a single region, i.e., uniform density and basis weight, cellulosic fibrous structure prior to final drying. The final drying is usually accomplished by a heated drum, such as a Yankee drying drum.
One of the significant improvements to the manufacturing process is the use of through-air-drying to replace conventional press felt dewatering. Through air drying yields significant improvements in consumer products. In through-air-drying, like press felt drying, the web begins on a forming wire which receives an aqueous slurry of less than one percent consistency (the weight percentage of fibers in the aqueous slurry) from a headbox. Initial dewatering takes place on the forming wire. From the forming wire, the web is transferred to an air pervious through-air-drying belt. This xe2x80x9cwet transferxe2x80x9d typically occurs at a pickup shoe (PUS), at which point the web may be first molded to the topography of the through air drying belt.
Through air drying yields structured paper having regions of different densities. This type of paper has been used in commercially successful products, such as Bounty paper towels and Charmin and Charmin Ultra brands of bath tissues. Traditional conventional felt drying does not produce the structured paper and its attendant advantages. However, it has been desired to produce structured paper using conventional felt drying at speeds approaching that of the through air dried systems.
Attempts have been made utilizing a conventional felt having a patterned framework thereon for imprinting the embryonic web. Examples of these attempts in the art include commonly assigned U.S. Pat. No. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; U.S. Pat. No. 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; U.S. Pat. No. 5,609,725, issued Mar. 11, 1997 to Phan; U.S. Pat. No. 5,629,052, issued May 13, 1997 to Trokhan et al.; U.S. Pat. No. 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; U.S. Pat. No. 5,674,663, issued Oct. 7, 1997 to McFarland et al.; and U.S. Pat. No. 5,709,775 issued Jan. 20, 1998 to Trokhan et al., the disclosures of which are incorporated herein by reference.
Other attempts have been made by transporting a paper web on a separate imprinting fabric and compressing the combination in a compression nip formed between two rolls. U.S. Pat. No. 4,421,600 issued Dec. 20, 1983 to Hostetler discloses an apparatus having two felts, three pressing operations, and a separate woven imprinting fabric. In Hostetler the web is transported on the imprinting fabric through the pressing operations before being delivered to the Yankee dryer.
Another such attempt in the art is illustrated by U.S. Pat. No. 4,309,246 issued Jan. 5, 1982 to Hulit et al. Hulit et al. describes three configurations where a nip is formed between two rolls. In each configuration, a paper web is carried on an imprinting fabric having compaction elements defined by knuckles formed at warp and weft crossover points. The imprinting fabric, web and a felt are compressed between the rolls.
Each of the aforementioned attempts in the art, requires a complex nip system in order to bring the imprinting fabric/paper web combination into contact with a dewatering felt. These systems create very expensive propositions for retrofitting existing conventional machinery, as additional space, drives, etc. are typically required to add the separate felt loop. What""s more, in order to sufficiently dewater the paper web, the systems are required to operate at lower speeds than through air dried systems.
Commonly assigned U.S. Pat. No. 5,637,194 issued Jun. 10, 1997 to Ampulski et al., the disclosure of which is incorporated herein by reference, discloses an alternative paper machine embodiment where a first dewatering felt is positioned adjacent a face of the imprinting member as the molded web is carried on the imprinting member from a first compression nip formed between two pressure rolls and a second dewatering felt to a second compression nip formed between a pressure roll and a Yankee drying drum. The imprinting member imprints the molded web and carries it to the Yankee drying drum. The presence of the first felt adjacent the imprinting member at the two compression nips results in additional water removal from the web prior to transfer to the Yankee drum.
The present invention provides a web patterning apparatus suitable for making structured paper on a conventional papermaking machine without the need for an additional dewatering felt or compression nip. The invention provides a web patterning apparatus capable of dewatering a paper web using conventional felt dewatering techniques with a single compression nip system while operating at speeds approaching that of through air dried systems.
The invention comprises papermaking apparatus and process for removing water from a cellulosic web. The papermaking apparatus comprises an imprinting member having an absolute void volume that enables a hydraulic connection to be formed between a cellulosic web and a capillary dewatering member when compressed in a nip. The absolute void volume is predetermined based on an estimate of the volume of water expressed from the cellulosic web at the nip. For the present invention, the ratio of the volume of water expelled from the web to the absolute void volume of the imprinting member is at least about 0.5.
The nip can be formed between first and second rolls juxtaposed coaxially. The cellulosic web is carried on the topside of the imprinting member. The cellulosic web and imprinting member are interposed in the nip such that the top surface of the cellulosic web is in contacting relationship with the periphery of the first roll. In the nip, the backside of the imprinting member is in contacting relationship with the top surface of a capillary dewatering member while the back surface of the capillary dewatering member is in contacting relationship with the periphery of the second roll. The nip compresses the paper web, the imprinting member, and the capillary dewatering member. Water expelled from the web passes through the imprinting member to the capillary dewatering member forming a hydraulic connection therebetween.